In IEEE 802.16e Uplink Partially Used Subchannelization (“PUSC”) mode, the minimal signal unit for receiver processing is a tile. A tile comprises four consecutive tones in the frequency domain and three consecutive Orthogonal Frequency Division Multiple Access (“OFDMA”) symbols. Six tiles chosen according to a pseudo random hopping sequence comprise a subchannel. A collection of subchannels used to transmit to a particular user is called an allocation. The OFDMA tiles hop around in the frequency-time grid to facilitate tone-hopping for interference mitigation. This hopping pattern is unique for each cell. Space Division Multiple Access (“SDMA”) is used in some wireless communication systems for optimizing the radio spectrum. For example, SDMA allows for channel separation to be obtained with users occupying the same time/frequency resources.
In a wireless communication system without SMDA, the tile hopping pattern is sector or cell dependent. Therefore, the interference seen is “average out”. However, in systems utilizing SDMA, two or more users occupying the same time/frequency resources are “supposed” to be separated by antenna array technology such as beam-forming, beam-steering, spatial interference cancellation, and the like. The four pilot symbols existing in a tile are simply binary-phase key shifting (“BPSK”) symbols. Therefore, there are only a total of 16 possible different pilot sequences.
If both users either have the same pilot sequence or the inverse sequence in a tile, one user can completely interfere with the other user for that tile. In other words, in the case of two users sharing one tile in a sector, the chance that the shared tile will be interfered with is ⅛. This can potentially limit the SDMA application. A similar problem exists in Adaptive Modulation Coding (“AMC”) mode, though not to as great an extent, where the chance that one bin (the minimum signal unit for receiver processing) is completely interfered with is 1/32.
One digital transmission technique used in IEEE 802.16e systems is Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a digital transmission technique in which a signal is split into several narrowband subchannels at different frequencies. When modulating and demodulating signals, OFDM minimizes the inter-subchannel interference and inter-symbol interference among the subchannels and symbols of the data stream. To obtain robust performance in poor signal conditions, Forward Error Correction (FEC) typically is used in conjunction with OFDM. In telecommunications, for example, FEC refers to a system of error control for data transmission where the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. FEC is implemented by adding redundancy to the transmitted information using some sort of coding or algorithm.
One type of OFDM receiver that employs FEC incorporates Low-Density Parity-Check (“LDCP”) codes. FIG. 1 is a block diagram that depicts a conventional OFDM communication system 100 utilizing LDPC as a means of FEC. An LDPC code is an error correcting code that provides a more reliable method of transmitting a message over a noisy transmission channel. LDPC uses a sparse parity-check matrix that is randomly generated and subject to sparsity constraints.
The system 100 can include a transmitter 102 which sends wireless signals via a channel 104 to a receiver 106. The transmitter 102 includes an LDPC encoder 108 which encodes bits of information. The encoded information bits are interleaved and mapped to Quadrature Amplitude Modulation (“QAM”) symbols in module 110. As known, interleaving generally scrambles the sequential order of the data stream according to a known pattern. The data stream can be interleaved with respect to time, frequency, or both time and frequency.
The resulting QAM symbols generated in module 110 are processed using an Inverse Fast Fourier Transform (“IFFT”) in module 112 to generate an OFDM symbol. Module 112 further adds a cyclic prefix to each OFDM symbol. The resulting signal can be transmitted via channel 104, i.e., as a wireless signal.
The receiver 106 includes a timing module 114 which selects samples to be processed using a Fast Fourier Transform (“FFT”) in module 116. The resulting signal is demodulated in demodulator 118. Functions including, but not limited to, channel estimation, equalization, automatic frequency control (“AFC”), and bit-log-likelihood ratio (LLR) generation also can be performed in demodulator 118. The bit-LLRs are de-interleaved, or descrambled, in de-interleave module 120. The de-interleave module 120 effectively reverses the interleaving process performed by module 110 to recover the proper data order. The resulting signal is provided to the LDPC decoder 122 where information bits are recovered.
One of the problems with the system 100 discussed above is that it only uses pilot symbols for separating the desired and interfering signals. If pilot symbols on a particular tile happen to coincide with either the pilot sequence or the inverse pilot sequence of an interfering signal, the interfering signal cannot be separated from the desired signal. This will degrade the performance of the whole receiver.
Therefore a need exists to overcome the problems with the prior art as discussed above.